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culture. Comparisons of the effects of these manipulations to the normal fate maps
demonstrated that cell fates depend on their environmental context. Spemann and
Mangold found that in tissue transplants, the dorsal lip of the blastopore self-
differentiates as axial mesoderm and induces the overlying ectoderm to form neural
tissue. Because of these properties, they named this tissue the Organizer.
Subsequently, tissues with Organizer properties were discovered at the onset of gas-
trulation in all vertebrate embryos. The fact that Organizer tissues function across
phyla boundaries indicated that a common mechanism acts in all vertebrates to
induce neural tissue, despite the obvious morphological differences between the
embryos at these stages. The discovery that Chordin and Noggin are expressed in
the amphibian Organizer, the teleost shield and the amniote node and streak con-
firmed this prediction and transformed our understanding of how the vertebrate
body plan is constructed. Chordin and Noggin are both secreted antagonists of the
Bone Morphogenetic Proteins (BMP), a subclass of the TGF-β superfamily that
ventralizes the embryo (Piccolo et al. 1996 ; Zimmerman et al. 1996 ). Thus, instead
of providing a signal that induces dorsal fates, signals from the organizer the act to
block ventralizing signals.
Experimental embryologists also identified the tissues that induce the Organizer,
called the Nieuwkoop Center, as discussed above. In amphibians, the Nieuwkoop
Center is located in the dorsal vegetal blastomeres, which are fated to become gut
endoderm, and other vegetal cells induce ventral mesodermal cell types. In teleosts
and amniotes, the Nieuwkoop Center is located in extraembryonic tissues, including
the YSL, PMZ and the mammalian VE. Signals from these tissues induce all meso-
dermal and endodermal cell types in the overlying blastoderm, including the shield
and node. The combined action of the Nieuwkoop Center and the Organizer explains
how development of the germ layers is coordinated. Therefore, the reason the foregut
always forms in the head is because prior to gastrulation, precursors of the foregut
induce organizer tissue, which induces and patterns the brain and head. In teleosts
and amniotes, the foregut precursors and the organizer tissues are both induced by
signals emanating from extraembryonic tissues, but the result is the same: coordi-
nated development of organs within the three germ layers.
7.6 Identification of Mesoderm Inducing Signals
The fact that extraembryonic tissues of teleosts, birds, and mammals have the same
mesoderm inducing capacity as the vegetal pole cells in amphibians suggests that
a common mechanism is responsible for inducing the germ layers in all verte-
brates. The embryological studies that identified the sources of the signals that
induce the germ layers provided the experimental framework for searching for the
endogenous mesoderm-inducing factors. Other mesoderm inducing factors were
identified through genetic studies in the mouse and zebrafish. The results from this
work demonstrated that a common mechanism acts in all vertebrates to specify the
germ layers.
W. Tseng et al.